Millions of taxpayers breathe a big sigh of relief in mid-April after they finish their tax returns and pay any tax they owe. For most of them, the promise of not having to worry about dealing with the hassle of preparing and filing returns with the Internal Revenue Service for almost another year takes a huge burden off their shoulders.

Yet what many people don't realize is that they might need to make an additional payment to the IRS as little as two months later. If you don't meet the guidelines to avoid doing so, then you might need to make quarterly estimated tax payments or else face the prospect of owing penalties in the future.

With big changes in taxes coming for the 2018 tax year as a result of tax reform, even those who were exempt from needing to pay quarterly estimates in the past might find themselves in a different situation this year.

Brick wall with carved rectangle saying Internal Revenue Service.

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Why would I have to make estimated tax payments?

The tax laws require you to pay taxes as you earn or otherwise receive income throughout the year. Most people get the vast majority of their income from their jobs. For them, withholding of federal income tax from their paychecks usually covers enough of their eventual tax burden that they don't have to make additional estimated tax payments to avoid the penalties that the IRS would otherwise impose.

However, if you don't have enough tax withheld through payroll withholding, then you'll need to make quarterly estimates in order to make up the difference. In addition, those who don't have traditional jobs and instead earn self-employment income will almost certainly need to make estimated tax payments in order to meet their tax obligations throughout the year. Those payments are due in four installments: on April 15, June 15, and Sept. 15 this year, and Jan. 15 of the following year.

Safe harbors that keep you from having to make estimates

There are two threshold amounts that you'll want to consider in deciding whether you should make estimated tax payments. First, if you expect to owe taxes of less than $1,000, then you generally won't have to pay quarterly estimates. Second, even if your tax is above $1,000, you won't owe a penalty if you pay at least 90% of your tax for the current year through withholding. Only if neither of those situations applies will you be required to make quarterly estimated tax payments.

Even in situations in which you'd otherwise have to make quarterly estimates, there's another exemption that can apply. The IRS gives relief to those who see a big rise in their tax liability from one year to the next by allowing taxpayers to meet the requirements by paying 100% of the tax they owed in the previous year. If you have at least that amount withheld, then you won't owe penalties. But for most taxpayers, the 100% figure rises to 110% of previous-year taxes if their income exceeds $150,000.

What if I don't make quarterly estimates when I should?

If you fail to make estimated tax payments when you're required to, you'll owe a penalty on top of your tax liability. The amount of the penalty is based on the amount you should have paid and prevailing interest rates at the time of the underpayments. For instance, throughout 2017, underpayment penalty rates were 4%, meaning that you would owe a penalty of $4 for every $100 you should have paid as a quarterly estimate on April 15, 2017, but waited until April 15, 2018, to pay. Penalties are assessed for each day that a given payment is late, with a pro-rata fraction of the annual penalty rate charged.

The calculations to figure the penalty are complicated, especially because there are some alternative methods that can result in a smaller penalty in certain situations.

Those who don't earn income in steady streams throughout the year are allowed to make estimated tax payments that aren't equal in size throughout the year, reflecting instead their relative income level for each period. For instance, if you make most of your money during the last three months of the year, then your estimated tax payments would be smaller for April 15, June 15, and Sept. 15, but be much larger for the following Jan. 15, which covers that high-earnings period.

Be smart about your taxes

If you need to make estimated tax payments, check out the IRS website and get a copy of Form 1040-ES today. Even if you discover that you should have made your first installment on April 15 and might not even be in time to make your June 15 payment, every day you wait could cost you another day's worth of penalties.

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